Let's take a more detailed look at how the volume controls work. This amplifier has a Volume knob and a Master Volume knob. This is a common setup on most tube amplifiers. The Volume knob is sometimes called "Gain" or "Preamp". That is because you are powering the signal that will be powered by the power tubes controlled by the Master Volume.

You'll notice we are only giving a demo of the Volume knob here and leaving the Master Volume knob in the same position. The Master Volume increases the volume of the signal you are shaping with the Volume knob. We don't increase the Master Volume here because of the recording levels on our video.

The most important thing to understand is that the Volume knob controls the gain in the signal and causes it to break up and distort more as you increase the volume level. This is overdrive and is also the sound that most overdrive pedals are trying to capture.